This is a study of the forces and interactions which determine the shapes of animal cells and which drive their dynamic mechanical functions. A new biophysical method has been developed to measure the local deformability of adherent cells in culture. A measured force is applied to a probe (2 microns in diameter) which makes a small indentation of measured depth (0.1 microns to greater than 1 micron) in the dorsal surface of a cell. Since the forces which determine the shape of a cell must also resist its deformation, measurements of deformability provide information about determinants of shape. We will use our measurements to study the role of intracellular cytoskeletal and contractile structures such as microtubules and microfilaments and extracellular components such as fibronectin and collagen which govern cellular morphology. We will also investigate the role of these structures and interactions in the mechanisms of physiological processes which involve large changes in cell shape, e.g. mitosis, transformation, and differentiation.